1. Field of the Invention
The present invention relates to a measurement apparatus, an exposure apparatus, and a device fabrication method.
2. Description of the Related Art
A projection exposure apparatus has conventionally been employed to fabricate, e.g., a semiconductor device and liquid crystal display device by using photolithography. The projection exposure apparatus transfers a pattern formed on a mask (reticle) onto, e.g., a wafer via a projection optical system.
In recent years, to attain further micropatterning of semiconductor devices, the practical application of a projection exposure apparatus (EUV exposure apparatus) using EUV (Extreme UltraViolet) light (wavelength: about 10 nm to 15 nm) having a wavelength shorter than that of ultraviolet light is under study. Since light absorption is great in various substances in the EUV wavelength range, a dioptric system using light refraction is impractical. For this reason, the EUV exposure apparatus uses a catoptric system using light reflection as the projection optical system. To exactly transfer the pattern of a mask at a predetermined magnification, it is necessary to suppress the wavefront aberration of the projection optical system to λ/14 (=0.96 nm) rms or less on the basis of the Marechal standard.
Accurate control of a projection optical system on an exposure apparatus requires a technique of accurately measuring the wavefront aberration of the projection optical system. A lateral shearing interferometer has conventionally been known as a measurement apparatus which measures the wavefront aberration of the projection optical system used for the exposure apparatus. Techniques associated with this apparatus are disclosed in Japanese Patent Laid-Open Nos. 2006-332586 and 2006-196699.
Unfortunately, a laser produced plasma light source (LPP) and discharge produced plasma light source (DPP), which are often used as light sources of EUV exposure apparatuses, emit light beams with low directivities. This makes it very difficult to selectively converge such a light beam only on a reflection unit which generates an ideal spherical wave. Since even a region (light-shielding region) other than the reflection unit is irradiated with the light beam, a signal noise ratio (S/N ratio) necessary for wavefront aberration measurement cannot sometimes be obtained due to the light beam reflected by the light-shielding region. In other words, the light beam reflected by the light-shielding region acts as noise of an interference pattern. This makes it impossible to accurately measure the wavefront aberration of an optical system to be measured.